Silicon materials are widely used as optical lenses and materials for windows because they have excellent transmittance over a wide range from a near-infrared region to a terahertz (THz) region (wavelength: about 1.1 μm to 10 mm), excellent workability, and easy availability.
For example, in Non-patent document 1 an infrared window having a transmission spectrum as shown in FIG. 9 is described.
The infrared window described in Non-patent document 1 shows an excellent linear transmittance of about 55% in the wavelength band ranging from 1.2 to 6 μm as shown in FIG. 9, and is expected to be desirably used in this wavelength band.
On the other hand, in recent years, research and development for an infrared detector that detects an infrared ray emitted from a human body and senses the human body based on the same have been actively made.
In such a type of infrared detector, it is desired to have high infrared transmittance in a wavelength band ranging from 8 to 12 μm around 10.6 μm because the infrared wavelength corresponding to around 37° C. which is a temperature of a human body (body temperature) is about 10.6 μm.
In Patent document 1, an infrared optical device obtained by processing polycrystalline silicon having a purity of nine nines (9N:99.9999999%) or higher produced by a CVD method into a lens shape or a filter shape is proposed.
In this Patent document 1, a mixed gas of a high-purity silicon compound such as mono-silane (SiH4) and hydrogen is heated to 900 to 1200° C. to cause a hydrogen reduction by a CVD method, and thereby polycrystalline silicon having a high purity of 9N is produced. Then, an infrared optical device with an anti-reflection coating, having a transmittance of 90% or higher at a wavelength of 9 μm is obtained.
Patent document 1: Japanese Patent Laying-Open No. 05-60901 (claim 1, Paragraph No. [0009], Table 1)
Non-patent document 1: SIGMA KOKI CO., LTD. WEB catalogue “infrared windows-silicon” [online], [searched on Aug. 11, 2008], INTERNET <URL:http://www.sigma-koki.com/B/Windows/WindowsSilicon/OPSI/OPSI.html>